The fine grain super-plasticity under the combined loading conditions was investigated experimentally by applying the combined loads of axial force and torque to the thin-walled tube specimen of 5083 aluminum alloy. Moreover, a set of constitutive equations for the fine-grained super-plastic materials was proposed on the basis of these experimental results and the classical (mathematical, phenomenological) theory of plasticity using the potential function. Here a new potential function was proposed by incorporating the second and third invariants of stress deviator. The proposed constitutive equations could predict well the mechanical behaviors of 5083Al alloy subjected to the combined loading conditions under the wide ranges of temperature and strain rate.